The present invention relates to a method for developing an edge stress in a windshield in a windshield bending furnace, wherein a glass sheet is supported on a ring mould and heated close to a softening temperature and, following a bending operation, the glass sheet is cooled at such a cooling rate that a compression stress is developed at least in the edge portion of the glass sheet.
The invention relates also to an apparatus for developing an edge stress in a windshield in a windshield bending furnace, the apparatus comprising a ring mould, the ring would support a glass sheet which is heated, bent and cooled.
A compression stress is desired to develop in the edge area of a windshield. This will make the glass more resistant to mechanical stresses, e.g., in robot-operated installations and to thermal stresses, applied to glass as a result of fluctuations of cold and hot air.
When producing long series of the same type of glass, the edge stress is provided by blasting cold air to the edge area after a bending operation. A problem in such mechanisms is that they have been designed for a certain type of glass and, when changing the glass type, they require the installation of a new blasting system, which takes a lot of time and halts production. Thus, this method cannot be applied to flexible production, wherein a glass design is changed often. One possible solution to the problem is set forth, e.g., in U.S. Pat. publication No. 4,871,385, which could also be applied to flexible production. However, this prior known mechanism is complicated and expensive.
An object of the invention is to provide a simple method and apparatus for developing an edge stress also in miscellaneous production.